Devices operating in mobile communication systems, such as cellular telephones and other wireless devices, are configured to communicate over wireless networks. It is often necessary to connect different signal paths of a mobile communication device to a common port, such as a common antenna port. For example, a mobile communication device includes a receiver and a transmitter that are typically connected to a common antenna through the common antenna port, for sending and receiving data and control signals over the wireless network. The signal paths and/or related signal path ports must be isolated from one another. Therefore, the signal paths include filtering devices formed by one or multiple band pass filters that have passbands corresponding to the signal frequency bands of the respective signal paths. For example, a duplexer which may be formed by two filtering devices then has two signal paths (e.g., a receive path from a common antenna to a receiver and a transmit path from a transmitter to the common antenna) with two corresponding filtering devices each being a band pass filters. Accordingly, the receiver is able to receive signals through a receive frequency passband, and the transmitter is able to send transmit signals through a different transmit frequency passband, while filtering out the other frequencies. More generally, a multiplexer may have multiple signal paths connected to a common port (two or more) with corresponding filtering devices, each formed by one or multiple band pass filters.
The receive and transmit signals may be radio frequency (RF) signals, for example, corresponding to various predetermined wireless communication standards, such as such as universal mobile telecommunications system (UMTS), global system for mobile communication (GSM), wideband code division multiple access (WCDMA), and Long-Term Evolution (LTE), for example. The communication standards identify separate bands for transmitting and receiving signals. For example, LTE is allocated various 3GPP bands, including bands 1, 3 and 7, where LTE band 1 provides an uplink frequency band of 1.920 GHz-1.980 GHz and a downlink frequency band of 2.110 GHz-2.170 GHz, LTE band 3 provides an uplink frequency band of 1.710 GHz-1.785 GHz and a downlink frequency band of 1.805 GHz-1.880 GHz, and LTE band 7 provides an uplink frequency band of 2.500 GHz-2.70 GHz and a downlink frequency band of 2.620 GHz-2.690 GHz. Accordingly, a duplexer for example operating in compliance with a 3GPP standard would include a filter having a passband within the corresponding uplink frequency band, and a filter having a passband within the corresponding downlink frequency band. Further, some new communication standards, such as LTE-A, require additional connections of transmit and receive signal paths to a common antenna port, for example, to provide carrier aggregation.
In RF communications, use of a common antenna requires matching the common antenna to respective ports of the multiple signal paths in their respective passbands to optimize signal transfer. This requirement drives the need for a matching circuit at the common antenna port that connects the different signal paths and the respective filters.
For conventional duplexers, there are two dominant matching techniques: quarter-wavelength matching and shunt inductor matching. According to the quarter-wavelength matching technique, the band pass filter having a passband that is higher in frequency is connected by a quarter-wavelength transmission line to a common antenna port, and the band pass filter having a passband that is lower in frequency is connected directly to the common antenna port with no quarter-wavelength transmission line. Thereby, the higher frequency band pass filter typically features a low input impedance in the passband of the lower frequency band pass filter which is transformed by the quarter-wavelength transmission line into a high impedance. Due to the high impedance, the higher frequency band pass filter does not substantially load the lower frequency band pass filter in corresponding passband of the lower frequency band pass filter. Since the lower frequency band pass filter typically already features a high impedance in the passband of the higher frequency band pass filter, no matching element is necessary. The quarter-wavelength matching technique typically requires the two filters to be in adjacent in frequency, such that the input impedance assumptions described above are valid.
In the shunt inductor matching technique, a shunt inductor is connected at a common connection point (common port or common antenna port) of the two band pass filters having corresponding passbands. Each band pass filter typically features a capacitive input impedance in the passband of the other band pass filter, which is transformed into a high impedance by the shunt inductor. Therefore, the band pass filters do not load each other in their respective passbands. The shunt inductor matching technique is more efficient when the passbands of the two band pass filters are further apart in frequency, such that the input impedance of each band pass filter does not change significantly due to acoustics in the passband of the other band pass filter.
The shunt matching technique may also be applied to a multiplexer with three or more band pass filters connected to a common antenna. In the passband of one band pass filter, the sum of capacitive input impedances of all other band pass filters in the multiplexer is transformed into a high impedance by the shunt inductor. This condition is typically met when the passbands of the band pass filters are within a relatively small frequency band (e.g., about 20 percent of the absolute frequency). For band pass filters having passbands distributed more broadly, a degradation is usually observed since frequency dependency of the shunt inductor impedance and the capacitive input impedance of each of the band pass filters does not match, and therefore the input impedances have to be selected sub-optimally. Further, adding additional band pass filters degrades overall performance, since more energy is stored in the matching circuit, resulting in occurrence of more losses.